Get join model from has_many_through relation

Question

Hi,

In my Rails app I have a has_many_through relation. I want to use the join model/table to store some data about the relation (how many times that particular relation is used, to be specific).

I'm writing an add-method on one of my classes which should check for any existing relation with the subject, update a counter on the relation if it exists or create it if it doesn't exist.

Example:

CoffeeDrinker is related to Coffee through Cup. Every time CoffeeDrinker takes a sip a counter on that particular Cup should be incremented. The first time CoffeeDrinker takes a sip, the Cup should be created and the counter initialized.

What is the easiest and/or most correct way to get a hold on the relation object?

Answer 1

Either I'm not understanding your question or you're going to be surprised how obvious this is. You will have defined the relationship as so:

#coffee_drinker.rb
has_many :cups
has_many :coffees, :through => :cup

#cup.rb
belongs_to :coffee
belongs_to :coffee_drinker

#coffee.rb
has_many :cups
has_many :coffee_drinkers, :through => :cup

coffee_drinker.cups
coffee_drinker.coffees

coffee.cups
coffee.coffee_drinkers

#coffee_drinker.rb
after_save :update_cups_drunk

def update_cups_drunk
  cups.find_by_coffee_id(coffee_id).increment!(:count)
end

#maybe you don't have access to the coffee_id

def update_cups_drunk
  cups.find_by_coffee_id(coffees.last.id).increment!(:count)
end